Method and system of flow control in multi-hop wireless access networks

ABSTRACT

A system, method and relay transceiver for multi-hop wireless communications. In the relay, there is a receiver for adapted to receive packets on a first wireless link collectively destined for at least one mobile terminal. The transmitter buffers them and then transmits the packets on a second wireless link. A flow control function is implemented to provide flow control over the first wireless link to limit an amount buffered in the relay transceiver for a given mobile terminal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/894,037, filed Jul. 20, 2004, the disclosure of which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to wireless multi-hop access networks.

BACKGROUND OF THE INVENTION

In conventional cellular wireless access networks, a cell is covered bya BTS (base station transceiver) and all mobile terminals communicatewith the BTS directly. With the addition of relay transceivers,hereinafter simply “relays”, a multi-hop network including the BTS,relay nodes and mobile terminals, is set up. With relay nodes involved,the coverage of a wireless access network is improved. In such awireless access network, there may be multiple routes for thecommunication between a terminal and network to take. For example aterminal can communicate directly with the BTS or indirectly via one ormore relay nodes.

An example of a two-hop scenario is shown in FIG. 1. Shown is a basestation 10 having a coverage area 12, and a relay 16 having a coveragearea 18. There is a link 22 between the base station 10 and the relay 16which is typically a high capacity and very reliable wireless link.Shown are a number of mobile terminals 14 communicating directly withthe BTS 10, and a number of mobile terminals 20 communicating with therelay 16 which relays communications for these mobile terminals to andfrom the BTS 10.

In such a network, the fixed relay node 16 is added to improve thecoverage in the edge of a cell/BTS. Since the relay is a fixed node, thechannel between the BTS 10 and the relay can be a high quality channelimplemented using any one of many advanced channel technologies, such asMIMO, which can provide improved capacity. However, the quality of thechannel between the relay node 16 and a mobile terminal 20 is typicallylower and less stable due to the mobility of the mobile terminal.Because of this, it is possible that data will accumulate in the relayafter having been transmitted over the reliable channel between the BTSand the relay. This requires that the relay node have a significantbuffer capacity, particularly in the case where a long delay bound isallowed, and there are a lot of mobile terminals that are served by therelay.

SUMMARY OF THE INVENTION

According to one broad aspect, the invention provides a relaytransceiver comprising: a receiver adapted to receive packets on a firstwireless link collectively destined for at least one mobile terminal; atransmitter adapted to transmit the packets on a second wireless link; abuffer adapted to buffer the packets between their receipt andtransmission; a flow control function adapted to participate in flowcontrol over the first wireless link to limit an amount buffered in therelay transceiver for a given mobile terminal.

In some embodiments, the flow control function is adapted to transmit amessage when the number of packets reaches a threshold.

In some embodiments, the threshold is selected to substantiallyguarantee the buffer will be emptied within a predetermined time.

In some embodiments, the threshold is based on a R Tc, where Tc is thepredetermined time, R is a minimum data rate on the second wirelesslink, and α is an average availability of the second wireless link fortransmitting to a given mobile terminal.

In some embodiments, flow control function is adapted to allow thebuffer to be emptied prior to completing a handoff.

In some embodiments, flow control function is adapted to allow thebuffer to be emptied prior to a mobile terminal generating a layer 2NACK the relay being adapted for use with mobile terminals that areconfigured to delay generating layer 2 NACKs for a predetermined time.

According to another broad aspect, the invention provides a systemcomprising: a first transceiver and a relay transceiver according toclaim 1 transceiver; wherein: the first transceiver is adapted totransmit packets to the relay transceiver on the first wireless link,each packet being destined for one of said at least one of mobileterminal; the relay transceiver is adapted to receive packets from thefirst transceiver on the first wireless link, to buffer the packets, andto transmit the packets on a second wireless link; and the firsttransceiver and relay transceiver perform flow control to limit anamount buffered in the relay transceiver for a given mobile terminal.

In some embodiments, the first transceiver and relay transceiver performflow control to limit size of the buffer in the relay transceiver for agiven mobile terminal by: the relay transceiver transmitting a messageto the first transceiver when an amount buffered for a given mobileterminal reaches a predetermined threshold.

In some embodiments, in response to receiving the message, the firsttransceiver stops transmitting packets to the relay transceiver for thegiven mobile terminal.

In some embodiments, the threshold is selected to substantiallyguarantee the buffer will be emptied within a predetermined time.

In some embodiments, the threshold is based on a R Tc, where Tc is thepredetermined time, R is a minimum data rate on the second wirelesslink, and alpha is an average availability of the second wireless linkfor transmitting to a given mobile terminal.

In some embodiments, a system is further adapted to: initiate a handoffto a second transceiver; executing the handoff after giving the relaytransceiver time to empty its buffer.

In some embodiments, the flow control is performed to substantiallyguarantee the buffer will be able to be emptied prior to completing thehandoff.

In some embodiments, a system for use with a mobile terminal adapted todelay sending a layer 2 NACK due to packet mis-ordering the systemfurther adapted to transmit one of a pair of consecutive packets for agiven mobile terminal directly from the first transceiver, and totransmit a second of the pair of consecutive packets from the firsttransceiver via the relay transceiver; wherein the threshold is selectedto substantially guarantee the buffer can be emptied before the mobileterminal is expected to generate a layer 2 NACK due to mis-ordering ofreceived packets.

According to another broad aspect, the invention provides a mobileterminal adapted to receive packets directly from a first transceiverand to receive packets indirectly from the first transceiver via a relaytransceiver, the mobile terminal being further adapted to: detect a gapin a received packet flow; upon detecting a gap in the received packetflow, wait a period of time for the gap to be filled before transmittinga control message indicating occurrence of the gap.

In some embodiments, the period of time set to be long enough for arelay transceiver to empty its buffer.

In some embodiments, the message is a layer 2 NACK.

According to another broad aspect, the invention provides a base stationfor providing wireless communication to at least one mobile terminal viaa relay transceiver, the base station comprising: a flow controlfunction adapted to participate in flow control of packets transmittedto a given mobile terminal via the relay transceiver; wherein the flowcontrol function is selected to substantially guarantee the relaytransceiver will be able to send all buffered packets within apredetermined time.

In some embodiments, a base station further comprises: initiate ahandoff to a second transceiver; execute the handoff after giving therelay transceiver time to empty its buffer.

In some embodiments, the flow control is performed to substantiallyguarantee the buffer will be able to be emptied prior to completing thehandoff.

Another broad aspect provides a method involving, at a relay, receivingpackets over a first wireless hop forming part of a multi-hopcommunications path; at the relay, buffering the packets fortransmission on a second wireless hop forming part of the multi-hopcommunications path; at the relay, transmitting the packets on thesecond wireless hop; and performing flow control over the first wirelesshop to limit an amount buffered for transmission on the second wirelesshop.

In a first method, performing flow control over the first wireless hopto limit an amount buffered for transmission on the second wireless hopcomprises: the relay periodically reporting a buffer status an upstreamtransmitter of the packets; the upstream transmitter transmitting thepackets as a function of the buffer status so as to limit the amountbuffered.

In a second method, performing flow control over the first wireless hopto limit an amount buffered for transmission on the second wireless hopcomprises: when the amount buffered reaches a stop point, the relayreporting that this has occurred to an upstream transmitter of thepackets; when the amount buffered drops to a resume point, the relayreporting that this has occurred to the upstream transmitter of thepackets; the upstream transmitter refraining from transmitting newpackets to the relay in respect of the multi-hop communications pathbetween receipt of the stop point report and the resume point report.

In a third method, performing flow control over the first wireless hopto limit an amount buffered for transmission on the second wireless hopcomprises: the relay sending a first report to an upstream transmitterof the packets when amount buffered reaches a first stop point; inresponse to the first report, the upstream transmitter refraining fromtransmitting new packets for some time and then automatically resumingtransmitting new packets; if after resuming, the relay detects that theamount buffered is still above the stop point, the relay sending asecond report to the upstream transmitter in response to which theupstream transmitter stops transmitting new packets.

In a fourth method, performing flow control over the first wireless hopto limit an amount buffered for transmission on the second wireless hopcomprises: the relay maintaining an estimate of an expected throughputfor the second wireless hop and periodically reporting this to anupstream transmitter of the packets; the upstream transmitter of thepackets transmitting packets to the relay approximately following theexpected throughput.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the attached drawings in which:

FIG. 1 is an example of a multi-hop wireless network;

FIG. 2 is a transmission scenario illustrating how data buffered in arelay may be lost after a handoff;

FIG. 3 is a packet flow diagram showing how the use of relays may resultin unnecessary packet re-transmission;

FIG. 4 is a system level diagram showing flow control between a relayand a BTS, as provided by an embodiment of the invention;

FIG. 5 is an example transmission scenario showing flow control betweenthe relay and BTS, as provided by an embodiment of the invention;

FIG. 6 is a flowchart of functionality executed in a BTS, in accordancewith an embodiment of the invention;

FIG. 7 is a flowchart of functionality implemented in a relay, inaccordance with an embodiment of the invention;

FIG. 8 is a packet flow diagram showing how packet re-ordering isachieved in accordance with an embodiment of the invention so as toavoid unnecessary NACKs;

FIG. 9 is a block diagram of functionality implemented by a mobileterminal in accordance with another embodiment of the invention; and

FIG. 10 is a block diagram of a base station, relay and mobile terminal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first problem that may occur in a multi-hop wireless network such asthat illustrated in FIG. 1 is the accumulation of packets in the relaywhich may then need to be discarded when a handoff occurs. An example ofthis is shown in FIG. 2 which shows signalling between a mobile terminal30, a relay 32, a serving BTS 34 and a candidate BTS 36. During time 40,the serving BTS 34 is simply forwarding all the data it receives to therelay 32. This is being forwarded by the relay to the mobile terminal 30on an ongoing basis. One example packet transmission is indicated asoccurring during transmission interval 42. Sometime later, a handoffrequest 44 is generated by the mobile terminal 30. While this is goingon, the serving BTS 34 is still transmitting to the relay and as such attime interval 48 there is a larger number of packets which haveaccumulated. At some point later, a handoff confirmation 47 is sent tothe terminal 30. There is only a short time 50 following the handoffconfirmation during which the relay 32 can continue to transmit packetsto the mobile terminal 30. In the illustrated example, only a singlepacket is transmitted at the time interval 50. Then, at 52 the mobileterminal 30 starts communicating with the new candidate BTS 36 which nowbecomes the serving BTS. Now all of the data accumulated in the relay 32is discarded. The result is that the network will have to re-transmitthat data again to the terminal after the handoff in order to achievelossless transmission. This translates into resource waste and longerpacket delay. A schematic view of the network is shown generallyindicated at 50. The mobile terminal 30 is shown moving from thecoverage area 32 served by BTS 34 into the coverage area 35 of BTS 36. ABSC 40 is shown communicating with each of the BTSs.

With the employment of fixed relays, more than one route exists betweenthe network and a given mobile terminal. The terminal can communicatedirectly with the BTS, or via one or more fixed relays. Because of this,the route from the mobile terminal to the network can be dynamicallyselected based on real-time channel considerations. This can bedetermined by QoS considerations for example. For delay sensitivetraffic, the communication should normally be established between theterminal and BTS directly. For non-delay sensitive traffic, thecommunication can be established between the terminals and the relay ifthe link between the relay and the terminal is better than the linkbetween the BTS and the terminals.

A problem which may occur in systems allowing for dynamic routing isthat packets may arrive at the mobile terminal out of order. It ispossible that a BTS will send a new packet to a mobile terminal directlybefore the relay has finished sending out all of the packets that it hasbuffered for the terminal. This may result in a L2 ARQ NACK (automaticrepeat request negative ACK) being generated by the mobile terminalwhich will again result in unnecessary packet re-transmission. Anexample of this is shown in FIG. 3 which shows a terminal 40, a relay 42and a BTS 44. A first packet P1 46 is transmitted by the BTS 44 directlyto the terminal 40. Sometime later, the BTS 44 transmits packets P2,P348 to the relay 42. The relay 42 starts forwarding these to the terminal40, and transmits packet P2 50 to the terminal 40. Sometime later, andbefore P3 is forwarded by the relay 42, the BTS 44 is shown transmittingP4 52 directly to terminal 40. At this point, the terminal 40 willdetect a gap in the packet flow. In particular, packet P4 will have beenreceived immediately after packet P2 without having received packet P3.The terminal detects this gap and immediately issues the RLP NACK 54.This will trigger the re-transmission of packets by the BTS 44. Thiswill occupy transmission resources and will also result in increaseddelay.

It is noted that some sort of scheme is typically provided in multi-hopnetworks to avoid self-collision. For example, the relay and a BTS maybe scheduled time-wise so as not to transmit to the same receiversimultaneously. Some implementations may use other mechanisms, such asfrequency divisional multiplexing.

Also, the relays may be provided to extend the coverage of a cell.Alternatively, they are used for enhanced data rate coverage. In thiscontext, packets can be sent to mobile terminals directly and via therelay for regions served by the relay.

Referring to FIG. 4, shown is a high level system diagram of a multi-hopwireless network featuring flow control between the relay and the BTS.Shown is a base station controller 50 responsible for controlling threeBTSs 52,54,56 illustrated by their coverage areas. Each sector is shownin communication with a respective relay 60,62,64 also illustrated bytheir coverage areas. Each coverage area is served by a respectivetransceiver forming part of a base station. Base stations may servemultiple sectors, or single sector base stations may be employed. Whilethree BTSs are shown in the example of FIG. 4, this can be extended toan arbitrary number of BTSs. Each BTS in the example of FIG. 4 has arespective relay. More generally, some BTSs and/or sectors of BTSs haverelays and some of them may not have relays. A base station controller50 is shown communicating with the three BTSs 52,54,56 in this example.The BSC-BTS is a particular network hierarchy which may not be featuredin all wireless multi-hop networks. However, the BTSs and relays maystill be provided in other networks as described below.

In operation, the BSC 50 is shown communicating with the BTSs 52,54,56by using multicast transmission 70. This means that each BTS gets a copyof everything that goes to every BTS. Each BTS then communicates withthe respective relay on a respective wireless communication path 72 andexchanges flow control information 76. Finally, the relays are showncommunicating bi-directionally with terminals as indicated at 74. Asingle terminal 65 is shown in the diagram. Of course it is to beunderstood that in a practical implementation there would be a muchlarger number of terminals and that the number would be changingdynamically as users enter and leave the network or as users turn on andturn off their terminals. Preferably, the wireless links between theBTSs and the relays have relatively high reliability and high capacitycompared to the wireless links between the relays and the terminals.

It is noted that while multi-cast is described as an example tool forenabling handoff, not all handoff mechanisms require this. For examplethere are other mechanisms which make use of context transfer and softhandoff. The flow control mechanisms described herein apply to varioushandoff mechanisms and not just multi-cast.

Because the link from the BTS to the relay is more reliable than thelink from the relay to the terminal, data received from a BTS by a relaycannot always be immediately forwarded to a terminal. For this reason,the relays are provided with a buffering capacity. In accordance with anembodiment of the invention, flow control is implemented between the BTSand the relay to ensure that at any moment, the amount of buffered datadoes not exceed an upper bound.

In a preferred embodiment, the upper bound is determined based on thefollowing parameters:

a) R=minimum data rate deliverable from a relay to a terminal. This isusually the data rate that would be employed during a handoff of aterminal, and is a function of the set of coding and modulation schemesavailable. The minimum data rate would be the lowest data rate of all ofthe available schemes.

b) α=average resource availability during a scheduling interval. Thisrefers to the average amount of transmit capacity from a given relaythat can be dedicated to the particular receiver. This will usually takeinto account the capacity being occupied by other users.

c) Tc=buffer clear-up time. This is the amount of time to be madeavailable to clear-up the data buffer by sending out all of the buffereddata for the terminal. Setting Tc smaller will result in quickerhandoffs, as described below. Setting Tc too small may reduce thecapacity of the system during non-handoff use.

The upper bound B is then determined as follows:

B=αRTc.

In the above equation, αR is the average available throughput of a user.That amount multiplied by Tc will give the amount of data which willalways be transmittable during the clear-up time Tc assuming no furtherproblems arise.

Each relay maintains knowledge of the number of packets buffered for agiven mobile terminal at any given instant and is capable of knowingwhen the upper boundary is exceeded. However, since it is not possibleto have instantaneous signalling between the relay and the BTS, inpreferred embodiments the relay alerts the BTS when the buffer status ofthe terminal exceeds some smaller value, for example 75% of the upperbound. This will allow time for signalling to take place between the BTSand the relay. Preferably, signalling is performed using layer 2signalling messages between the relay and the BTS.

A very particular formula for determining an upper bound B has beenprovided above. As described below, the objective of this is to allowthe relay to transmit any data in its buffer before the hand-off iscomplete. Thus, more generally, any flow control method implementedbetween the BTS and the relay which results in there being enough timeto empty the relay buffer prior to handing off to another transceivercan be implemented. Various example flow control methods will now bedescribed.

In a first example flow control method, the relay reports the bufferstatus to the base station periodically. This will allow a tight controlover the amount of buffer data but involves a high overhead torepeatedly transmit the buffer status. The buffer status might forexample consist of the current amount of data or packets buffered.

In a second method, two thresholds for the buffer amount are set. One ofthe thresholds is a “stop point” and the other is a “resume point”. Whenthe buffer content reaches the stop point, the relay reports to the basestation that this has occurred. When the buffer amount drops to theresume point, this is again reported to the base station. The basestation will stop transmitting new data between receipt of the stoppoint report and the resume point report. This has the advantage oflower overhead than the first method described above.

In a third method, the relay sends a report when the buffer amountreaches a first stop point. The base station response to this bystopping transmission to the particular relay for that user for somenumber of frames which may or may not be fixed, for example N frames.After this wait, the base station automatically starts to transmitwithout receipt of a further report from the relay. If after resuming,the relay detects that its buffer status is above the stop point, afurther report is sent back to the base station and the base stationstops transmitting. This results in a low overhead still than the twoabove described methods.

In a fourth method, an estimate of αR_(k)′ is made for each terminal.The relay periodically reports this throughput and the base station onthe basis of this provides data to the relay approximately following theexpected throughput. Ideally this should result in the buffer notbecoming over filled in the relay. This has very low overhead but hasincreased complexity at the base station side.

Even more generally, the method involves at a relay, receiving packetsover a first wireless hop forming part of a multi-hop communicationspath; at the relay, buffering the packets for transmission on a secondwireless hop forming part of the multi-hop communications path; at therelay, transmitting the packets on the second wireless hop; andperforming flow control over the first wireless hop to limit an amountbuffered for transmission on the second wireless hop.

In a first method, performing flow control over the first wireless hopto limit an amount buffered for transmission on the second wireless hopcomprises: the relay periodically reporting a buffer status an upstreamtransmitter of the packets; the upstream transmitter transmitting thepackets as a function of the buffer status so as to limit the amountbuffered.

In a second method, performing flow control over the first wireless hopto limit an amount buffered for transmission on the second wireless hopcomprises: when the amount buffered reaches a stop point, the relayreporting that this has occurred to an upstream transmitter of thepackets; when the amount buffered drops to a resume point, the relayreporting that this has occurred to the upstream transmitter of thepackets; the upstream transmitter refraining from transmitting newpackets to the relay in respect of the multi-hop communications pathbetween receipt of the stop point report and the resume point report.

In a third method, performing flow control over the first wireless hopto limit an amount buffered for transmission on the second wireless hopcomprises: the relay sending a first report to an upstream transmitterof the packets when amount buffered reaches a first stop point; inresponse to the first report, the upstream transmitter refraining fromtransmitting new packets for some time and then automatically resumingtransmitting new packets; if after resuming, the relay detects that theamount buffered is still above the stop point, the relay sending asecond report to the upstream transmitter in response to which theupstream transmitter stops transmitting new packets.

In a fourth method, performing flow control over the first wireless hopto limit an amount buffered for transmission on the second wireless hopcomprises: the relay maintaining an estimate of an expected throughputfor the second wireless hop and periodically reporting this to anupstream transmitter of the packets; the upstream transmitter of thepackets transmitting packets to the relay approximately following theexpected throughput.

Referring now to FIG. 5, shown is a detailed signalling exampleinvolving the network elements in the example network of FIG. 4. For thepurpose of this example, it is assumed that BTS 52 is the serving BTS,BTS 54 is a candidate BTS, and that a terminal 65 is communicating viarelay 62 with the serving BTS 52. During a first transmission interval80, the serving BTS is transmitting packets to the relay 60 which arebuffered. At transmission interval 82, a packet is shown beingtransmitted from the relay 60 to the terminal 65. A handoff request isshown at 84. This is sent from the terminal 65 to the relay 60 and thenfrom the relay 60 on to the serving BTS 52. This information is alsosent to the candidate BTS 54 as indicated at 85. However, typically thiswould be sent via some other channel, for example via the BSC of FIG. 4.After receiving the handoff request, the BTS 52 stops sending any moredata to the terminal's relay 60. A handoff confirmation is generated at88. In a centralized system, a higher component such as a BSC (basestation controller) may generate the handoff confirmation 88.Alternatively, it can be generated by the serving BTS as shown in FIG.5. Following this, the relay 60 continues to transmit the contents ofits buffer towards the terminal 65. This is shown during transmissionintervals 92,96. The buffer becomes empty as indicated at 95. At 98, thecandidate BTS 54 becomes the serving BTS and starts to communicate withthe terminal 65. The amount of data in the data buffer is capped to anupper bound, for example the upper bound described above. However, moregenerally the flow control is implemented so that the buffer has time toclear-up before the candidate BTS 54 becomes the serving BTS. A clear-uptime 99 is indicated in FIG. 5. The buffer is expected to be emptiedafter expiry of the clear-up time 99. When the BTS sends the handoffconfirmation 88 to the terminal, it indicates an “action time” afterwhich the handoff will actually occur. The action time is set to belonger than the amount of time allocated to clear-up the buffer, namelyTc. The serving BTS 52 also indicates to the candidate BTS 54 the firstpacket sequence number for synchronization purposes. Since the servingBTS 52 knows the last packet that it transmitted to the relay 60, thenext packet will be the first packet to be transmitted by the candidateBTS 54. The first packet sequence number is used to indicate to thecandidate BTS 54 what this first packet is. Any appropriate mechanism ofinforming the candidate BTS of the next sequence number can be employed.Preferably, after the decision to perform a handoff has been made, butbefore the occurrence of the action time, the relay 60 gives higherpriority to the particular terminal being handed off. This assigns asmuch resources as possible to the terminal without impairing the QoS ofother terminals so that as much of the terminal's buffered data can betransmitted to the terminal during this time as possible.

Preferably, packets are multicast to the serving BTS and to BTSs whichare handoff candidates. If this is done then the new serving BTS canstart transmitting immediately after handoff since it will have thepackets. In embodiments with multicast, the candidate BTS will have thepackets before and after the first packet it will transmit. Innon-multicast, the system may simply start sending the candidate BTSpackets starting at the first packet it needs to transmit. In theseother methods there may be a slight scheduling delay when switching to anew base station. Multicasting can be done from a central device to anactive set of base stations, or to an anchor base station that forwardsthe packets to candidate base stations.

Transitions between communicating directly with the BTS andcommunicating indirectly with the BTS via the relay can be accommodated.Preferably, when a transition is made from the relay to the BTS or viceversa, this is instantaneous meaning a first packet may be sentdirectly, and a next packet via the relay or vice versa. As describedbelow preferably the mobile terminal is adapted to ignore gaps in packetsequence to allow them to be filled.

FIG. 6 is a flow chart of a method provided by an embodiment of theinvention for implementation in a BTS. The method begins at step 6-1with the start of transmitting to a relay. This can be the very start ofa session to a mobile terminal, or alternatively can indicate the startof transmitting following a handoff to that particular BTS. At step 6-2,flow control is implemented to cap the size of the buffer in the relaybeing used by the BTS. At step 6-3, upon receipt of a handoff request,the BTS stops sending data to the terminal's relay. An action time isset, for example by the BTS or BSC, to be long enough to clear-up thebuffer in the relay, as indicated at step 6-4. This step does not haveto be implemented in the BTS. For example the buffer clear-up time mightbe a predetermined value, or this value can be determined elsewhere. Theaction time is also sent to the terminal in a confirmation message. Inanother embodiment, the handoff is instigated by the network. In thiscase, the method of FIG. 6 applies with the exception that step 6-3involves receiving a handoff request from a control entity, orautonomously making the handoff decision.

FIG. 7 is a flowchart of a method implemented in a relay in accordancewith another embodiment of the invention. The method starts at step 7-1with the use of flow control to cap the size of the buffer and informthe BS of the buffer status. At step 7-2, the relay receives a handoffrequest from a mobile terminal, and forwards this on to the BTS. At step7-3, the relay empties out its buffer, preferably giving higher priorityto the terminal prior to handoff.

The functionality and embodiments described thus far has focused on thebehaviour of the relay and the BTS to solve the handoff problem.Preferably, in some embodiments the behaviour of the terminal is alsomodified to improve performance and capacity and to solve theunnecessary NACK problem. This will be illustrated first by way ofexample with reference to FIG. 8. This example uses the same BTS 52,relay 60 and terminal 65 used in the previous examples. A packet P1 100is shown being transmitted from the BTS 52 directly to terminal 65.Sometime later, packets P2,P3 102 are shown transmitted from the BTS 52to the relay 60. Packet P2 104 is then shown transmitted by the relay 60to the terminal 65. Sometime later, the BTS 52 transmits a packet P4 106directly to the terminal 65 prior to relay 60 having transmitted packetP3. At this point, the terminal 65 will detect a gap in the packet flow.In conventional systems, the terminal would immediately send an RLPNACK. However, in this embodiment, rather than sending an RLP NACK, theterminal sets a clear-up time 112 during which it is willing to wait forthe gap in the packet flow to be filled. Thus, sometime later the BTS isshown transmitting packet P5 112 to the terminal 65. Sometime later, therelay 60 sends packet P3 114 to the terminal 65. The terminal 65 nowfills in the gap in its packet flow. At this point, the terminal sendsthe packets P3 through P5 to upper protocol layers in the terminal. Thusgenerally, when a terminal receives a packet from a BTS with a sequencenumber higher than expected, the terminal should wait before issuing anRLP NACK preferably for a time equal to Tc. More generally still, when aterminal detects a gap in a packet flow, it should wait for some amountof time before transmitting a message to indicate that the gap hasoccurred.

FIG. 9 is a flowchart of behaviour of a mobile terminal to implement amethod such as exemplified in FIG. 8. The method begins either at step9-1 with the expiry of the clear-up timer, or at step 9-4 with thereceipt of a packet. After receipt of a packet, if the clear-up timer isrunning, yes path step 9-5, then a determination is made at step 9-6 ofwhether or not the received packet fills the gap. If it does not fillthe gap then the method returns to either step 9-1 or 9-4. If on theother hand, the packet does fill up the gap, yes path step 9-6, then thepackets are re-ordered at step 9-7, the time is reset at step 9-9, andthe packets are sent to the upper layer at step 9-11. If on the otherhand, the clear-up timer is not running, no path step 9-5, then a gapmay be detected at step 9-8. If no gap is detected, no path step 9-8,then the packets are sent to the upper layer at step 9-11. On the otherhand, if a gap is detected, then a clear-up timer is started at step9-10.

In the event that the clear-up timer does expire, and as such step 9-1occurs, the timer is reset at step 9-2, and a NACK is issued at step9-3. It can be seen that the only circumstance under which the NACK isissued is if a gap in the packet sequence is detected, and subsequent tothat gap being detected the clear-up timer has expired without the gapbeing filled.

In the flowchart of FIG. 9, it is assumed that multiple gaps can bedetected, and that for each gap that is detected a respective clear-uptime is maintained. If a given gap is not detected within thisrespective clear-up time, then the NACK is generated. In a simplerimplementation, only a single gap is tracked, and that single gap has asingle clear-up time within which the continuity of numbering ofreceived packets must be established notwithstanding whether or notthere might have been additional gaps created after the initial gap.

Referring now to FIG. 10, shown is a block diagram of an example systemimplementation for realizing the above described methods. The systemcomprises a base station controller (BSC) 100 communicating with twoBTSs, BTS1 102 and BTS2 104. A relay 112 is shown for extending thecoverage area of BTS1 102. It is to be understood that a given systemmight include further BTSs, and any of the BTSs might include a relay orrelays to extend their coverage area. Also shown is a single mobileterminal 126 communicating with BTS1 102 via the relay 112. Of coursethe number of mobile terminals in a given system will vary in an ongoingmanner. The internal details of BTS1 102 are shown at a very high level.It is also noted that the functionality shown in FIG. 10 is specific todownlink communications. However, most implementations would alsoinclude the capability to communicate in the opposite direction from themobile terminal up to the BTS. The BTS1 102 has a transmitter 110 andtransmit antenna 111. It is to be understood that any appropriateantenna technology can be employed. There might be multiple antennas forexample or just a single antenna. Furthermore, a given BTS might be amultiple sector BTS in which case the functionality for each sector isconsidered a respective transceiver. A flow control function 108 isshown. This is simply responsible for interacting with the relay to capthe size of the buffer in the relay. The BTS1 102 also has a buffer 106.This is for buffering packets received from the BSC 100 prior to theirtransmission to the mobile terminal 126 either directly or via the relay112. Also, in some embodiments packets are multicast to all basestations which are either actively serving a given mobile terminal, orare a candidate for handoff for the given mobile terminal. Thus, if BTS2104 is a candidate for handoff, then packets would be multicast to bothBTS1 102 and BTS2 104 for the mobile terminal 126. A particulararrangement of the functions in the BTS1 102 has been shown. It is to beunderstood that these functions can be implemented in any suitablecombination of hardware, software, firmware, etc. Also, while they areshown as distinct functional entities, they may be implemented as asmaller or larger number of distinct functional entities. The BTS wouldalso of course include much other functionality not shown.

The relay 112 is shown to have an antenna 114 for communicating with theBTS1 102, and an antenna 124 for communicating with mobile terminalssuch as mobile terminal 126. Depending upon the technology used tocommunicate between BTS1 102 and the relay 112, the number of antennas,and the shape, size and structure of the antenna 114 can be modifiedaccordingly. The same applies for antenna 124. It is noted that the linkbetween BTS1 102 and relay 112 may be implemented to be a highercapacity link and more reliable than the link emanating from antenna124. The relay 112 is shown to have a receiver 116 and a transmitter122. There is a flow control function 120 and a buffer 118. The buffer118 buffers packets between their receipt by the receiver 116 and theirtransmission by the transmitter 122. The flow control function 120 inCupertino with the flow control function 108 and the BTS1 102 caps thesize of the buffer 118. The functionality of the relay 112 can beimplemented in any suitable combination of hardware, software andfirmware, etc. A particular layout of the functional entities in a relay112 has been shown. More generally, this functionality can be providedin a smaller or larger number of functional entities.

The mobile terminal 126 is shown to include an antenna 128. As for theother components described above, depending upon the technology beingemployed for the wireless communications to the mobile terminal 126, anysuitable number and design of antennas 128 can be employed. The mobileterminal 126 has a receiver 130. The mobile terminal 126 is also shownto include layer 1 132 and modified layer 2 134 of its protocol stack.The mobile terminal would typically include other layers and otherfunctions not shown. The modified layer 2, in preferred implementations,does not generate the layer 2 NACK immediately upon detecting a gap in apacket flow, but rather waits for some amount of time in order to allowtime for the gap to be filled.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A mobile terminal adapted to receive packets directly from a firsttransceiver and to receive packets indirectly from the first transceivervia a relay transceiver, the mobile terminal being further adapted to:detect a gap in a received packet flow; upon detecting a gap in thereceived packet flow, wait a period of time for the gap to be filledbefore transmitting a control message indicating occurrence of the gap.2. A mobile terminal according to claim 1 wherein the period of time setto be long enough for a relay transceiver to empty its buffer.
 3. Amobile terminal according to claim 1 wherein the message is a layer 2NACK.